Mrercury 700 dropping boost
 

I am having issues with a 700 mercury dropping boost . Boost will go up to 7 to 8 lbs and then drop to 4 . The boost control bypass has been replace and checked it is good.. If we work the throttle sometimes it will come out of it. Any Ideas??
thanks

My AT had that issue and we traced it back to the motors "talking" across the electronics when we had the batteries on "both". When we had them set individually it never happened. Not a true answer on why but it fixed the issue.

mine has a battery isolater deal first boat i had with one . Its running triple 700 Its only running 2 alternators .I have been told the isolater takes care of the rest.

...and just another reason to not run your boat with battery switches on both!! It makes your boat faster! :-)

I'll add that to my list of reasons when the battery switches thread comes back again. LOL!

Any chance the belt is slipping at higher throttle settings???

the first thing i would do is replace the belt and check the tensiner.

I wouldn't think it would be intermittent then but I will check it out .thanks

I was recently looking over a 700sci engine. I noticed the belt tensioner was stamped "dayco" . I remember the 600sc and 800sc merc's, had GM tensioners off an LT4 vette. Even had the GM part # stamped on them still. 50 dollar tensioner at GM dealer, vs 400 bucks from Mercury. Same goes for the belts. Basic 10 rib belt, 30 bucks from parts store, or 120 bucks from merc.

Anyhow, any noticeable belt dust anywhere?

no and it is like a light switch when when it looses it

Have you had an IAC fail on this same engine in the recent past? The boost controller and IAC are on the same circuit/fuse and it is not uncommon for a failed IAC to cause damage to the PCM, which could be impacting the boost controller.

Just a thought,
GF?